1. Field of the Invention
The present invention relates to a process for the production of polyisocyanates containing iminooxadiazinedione groups.
2. Description of the Prior Art
Polyisocyanates containing iminooxadiazinedione groups (asymmetric trimers) are high grade raw materials, which may be used for the production of polyurethane lacquers and coatings (DE-A 19,611,849). These polyisocyanates are present as a subsidiary component in the well known polyisocyanates containing isocyanurate groups (symmetric trimers).
Isocyanate oligomers having a significantly increased iminooxadiazinedione content are described in DE-A 19,611,849. Their advantageous properties, for example, as a raw material for the manufacture of polyurethane lacquers and coatings, are described. For isocyanate oligomers having at least three NCO groups, poly(di)-isocyanates containing iminooxadiazinedione groups have the lowest viscosity.
DE-A 19,611,849 describes the use of hydrogen (poly)fluorides corresponding to the formula, M[nF.sup.- .multidot.(HF).sub.m ], wherein .sup.m /.sub.n is &gt;0 and M represents an n-charged cation or an n-valent residue, as a catalyst for isocyanate trimerization with preferential formation of iminooxadiazinedione groups. This process is disadvantageous requires it handling HF during the production of the catalysts, which usually starts from the corresponding fluorides, M[nF.sup.- ], wherein M is an n-charged cation.
The handling of HF restricts the technical feasibility of the entire process because particular precautions are required for handling, optionally anhydrous, hydrofluoric acid which greatly complicates the production of the catalyst. In addition, due to the corrosiveness of the material, certain requirements arise with regard to the selection of the reactors in which the catalyst may be produced and used. These circumstances restrict the widespread, safe performance of the isocyanate trimerization process when it is desired to form a high proportion of iminooxadiazinedione groups.
An object of the present invention is to provide a process that does not require hydrofluoric acid to be handled during the production of the catalysts, but still yields products having an elevated content of iminooxadiazinedione groups in the trimer mixture. The term "trimer mixture" means the sum of isocyanurate and iminooxadiazinedione groups. An elevated iminooxadiazinedione group content means products containing at least 30% of iminooxadiazinedione groups in the trimer mixture.
This object may be achieved with the process of the present invention described hereinafter, in which isocyanate trimerization is catalyzed by quaternary ammonium or phosphonium fluorides.